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CWE
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Improper Encoding or Escaping of Output

ID: 116Date: (C)2012-05-14   (M)2017-05-20
Type: weaknessStatus: DRAFT
Abstraction Type: Class





Description

The software prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.

Extended Description

Improper encoding or escaping can allow attackers to change the commands that are sent to another component, inserting malicious commands instead.

Most software follows a certain protocol that uses structured messages for communication between components, such as queries or commands. These structured messages can contain raw data interspersed with metadata or control information. For example, "GET /index.html HTTP/1.1" is a structured message containing a command ("GET") with a single argument ("/index.html") and metadata about which protocol version is being used ("HTTP/1.1").

If an application uses attacker-supplied inputs to construct a structured message without properly encoding or escaping, then the attacker could insert special characters that will cause the data to be interpreted as control information or metadata. Consequently, the component that receives the output will perform the wrong operations, or otherwise interpret the data incorrectly.

Likelihood of Exploit: Very High

Applicable Platforms
Language Class: All
Technology Class: Often
Technology Class: Database-Server
Technology Class: Often
Technology Class: Web-Server

Time Of Introduction

  • Architecture and Design
  • Implementation
  • Operation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
Integrity
Confidentiality
Availability
Access_Control
 
Modify application data
Execute unauthorized code or commands
Bypass protection mechanism
 
The communications between components can be modified in unexpected ways. Unexpected commands can be executed, bypassing other security mechanisms. Incoming data can be misinterpreted.
 

Detection Methods

NameDescriptionEffectivenessNotes
Automated Static Analysis
 
This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.
 
Moderate
 
 
Automated Dynamic Analysis
 
This weakness can be detected using dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.
 
  

Potential Mitigations

PhaseStrategyDescriptionEffectivenessNotes
Architecture and Design
 
Libraries or Frameworks
 
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
For example, consider using the ESAPI Encoding control [R.116.1] or a similar tool, library, or framework. These will help the programmer encode outputs in a manner less prone to error.
Alternately, use built-in functions, but consider using wrappers in case those functions are discovered to have a vulnerability.
 
  
Architecture and Design
 
Parameterization
 
If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
For example, stored procedures can enforce database query structure and reduce the likelihood of SQL injection.
 
  
Architecture and Design
Implementation
 
 Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies.
 
  
Architecture and Design
 
 In some cases, input validation may be an important strategy when output encoding is not a complete solution. For example, you may be providing the same output that will be processed by multiple consumers that use different encodings or representations. In other cases, you may be required to allow user-supplied input to contain control information, such as limited HTML tags that support formatting in a wiki or bulletin board. When this type of requirement must be met, use an extremely strict whitelist to limit which control sequences can be used. Verify that the resulting syntactic structure is what you expect. Use your normal encoding methods for the remainder of the input.
 
  
Architecture and Design
 
 Use input validation as a defense-in-depth measure to reduce the likelihood of output encoding errors (see CWE-20).
 
  
Requirements
 
 Fully specify which encodings are required by components that will be communicating with each other.
 
  
Implementation
 
 When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.
 
  

Relationships
This weakness is primary to all weaknesses related to injection (CWE-74) since the inherent nature of injection involves the violation of structured messages.
CWE-116 and CWE-20 have a close association because, depending on the nature of the structured message, proper input validation can indirectly prevent special characters from changing the meaning of a structured message. For example, by validating that a numeric ID field should only contain the 0-9 characters, the programmer effectively prevents injection attacks.
However, input validation is not always sufficient, especially when less stringent data types must be supported, such as free-form text. Consider a SQL injection scenario in which a last name is inserted into a query. The name "O'Reilly" would likely pass the validation step since it is a common last name in the English language. However, it cannot be directly inserted into the database because it contains the "'" apostrophe character, which would need to be escaped or otherwise neutralized. In this case, stripping the apostrophe might reduce the risk of SQL injection, but it would produce incorrect behavior because the wrong name would be recorded.

Related CWETypeViewChain
CWE-116 ChildOf CWE-896 Category CWE-888  

Demonstrative Examples   (Details)

  1. Consider a chat application in which a front-end web application communicates with a back-end server. The back-end is legacy code that does not perform authentication or authorization, so the front-end must implement it. The chat protocol supports two commands, SAY and BAN, although only administrators can use the BAN command. Each argument must be separated by a single space. The raw inputs are URL-encoded. The messaging protocol allows multiple commands to be specified on the same line if they are separated by a "|" character.
  2. Here a value read from an HTML form parameter is reflected back to the client browser without having been encoded prior to output.
  3. This example takes user input, passes it through an encoding scheme and then creates a directory specified by the user.

Observed Examples

  1. CVE-2008-4636 : OS command injection in backup software using shell metacharacters in a filename; correct behavior would require that this filename could not be changed.
  2. CVE-2008-0769 : Web application does not set the charset when sending a page to a browser, allowing for XSS exploitation when a browser chooses an unexpected encoding.
  3. CVE-2008-0005 : Program does not set the charset when sending a page to a browser, allowing for XSS exploitation when a browser chooses an unexpected encoding.
  4. CVE-2008-5573 : SQL injection via password parameter; a strong password might contain "&"
  5. CVE-2008-3773 : Cross-site scripting in chat application via a message subject, which normally might contain "&" and other XSS-related characters.
  6. CVE-2008-0757 : Cross-site scripting in chat application via a message, which normally might be allowed to contain arbitrary content.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

TaxynomyIdNameFit
WASC 22
 
Improper Output Handling
 
 
CERT Java Secure Coding IDS00-J
 
Sanitize untrusted data passed across a trust boundary
 
 
CERT Java Secure Coding IDS12-J
 
Perform lossless conversion of String data between differing character encodings
 
 
CERT Java Secure Coding IDS05-J
 
Use a subset of ASCII for file and path names
 
 
CERT C++ Secure Coding MSC09-CPP
 
Character Encoding - Use Subset of ASCII for Safety
 
 
CERT C++ Secure Coding MSC10-CPP
 
Character Encoding - UTF8 Related Issues
 
 

References:

  1. OWASP .OWASP Enterprise Security API (ESAPI) Project.
  2. Jeremiah Grossman .Input validation or output filtering, which is better?.
  3. Joshbw .Output Sanitization. 2008-09-18.
  4. Niyaz PK .Sanitizing user data: How and where to do it. 2008-09-11.
  5. Jeremiah Grossman .Input validation or output filtering, which is better?. 2007-01-30.
  6. Jim Manico .Input Validation - Not That Important. 2008-08-10.
  7. Michael Eddington .Preventing XSS with Correct Output Encoding.
  8. M. Howard D. LeBlanc .Writing Secure Code 2nd Edition. Microsoft. Section:'Chapter 11, "Canonical Representation Issues" Page 363'. Published on 2002.

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